ADVANCES
18 Scientific American, March 2021
WILSON R. RAMIREZIllustration by Thomas FuchsO P T I C S
Traveling
Photons
A laser-based system could
boost deep-space data transfer
A new laser technology could improve
the quality of deep-space communica-
tion, making it easier for humans to push
the boundaries of the final frontier.
Much of today’s space communication
relies on radio signals. But these diffract and
broaden as they travel, as does light or any
other electromagnetic wave. A radio beam
fired from the moon to Earth “would typi-
cally diverge to the size of a continent,” says
Peter Andrekson, a photonics researcher
at Chalmers University of Technology in
Sweden and co-author of a new study in
Light: Science and Applications. In contrast,
he notes, “a laser beam would diverge to
a two-kilometer radius or so.”
Catching enough of a spacefaring radio
signal from somewhere like Mars requires
a really big dish. nasa’s widest receivers
stretch 70 meters across, says Bryan Robin-
son, an optical communications engineer
at the MIT Lincoln Laboratory, who was not
involved in the study: “It’s like a football field
that’s on a gimbal pointing to Mars.”
Laser communication could work with
receivers about 20 centimeters across—the
size of a personal pizza—and condensed
laser beams can carry much more informa-
tion than radio. But laser signals are trans-
mitted at a lower power level, and process-
ing them once they are received requires
a daunting level of amplification.
The researchers’ new receiver manipu-
lates interactions between photons to mag-
nify an incoming signal without reducing its
quality, a technique called phase-sensitive
amplification (PSA). This approach is “very
interesting,” Robinson says, because
today’s amplifiers add distorting “noise.”
The experimental PSA system was sensi-
tive enough to receive an unprecedented
10.5 gigabits of information per second,noise-free, through a lab setup that mimics
the vacuum of deep space and adds diffrac-
tion to simulate distance. The next chal-
lenge will be to overcome distortion caused
by Earth’s atmosphere.
In 2013 the Lincoln Laboratory and
nasa successfully tested another type
of laser transmission between a spacecraft
and Earth. That method used a photon-
counting receiver, which tallies individual
light particles as they strike a detector.
It is extremely efficient for transmitting
data, which can be numerically encoded—
but the counter works only at –454 degrees
Fahrenheit. PSA receivers operate at
room temperature.
Despite the challenges, refining optical
communications systems such as these
would be “a pretty big deal,” says planetary
scientist Tanya Harrison, who was not
involved with either project. Harrison is
mapping Mars by satellite and has been
frustrated with the limitations of radio trans-
missions. Radio data currently travel from
Mars to Earth with all the speed and fidelity
of an early-1990s modem. A satellite orbiting
the Red Planet, Harrison says, “can take an
order of magnitude more data than it’s able
to actually send back. Basically we could be
doing a lot more science if we had optical
communications.” — Joanna ThompsonG E O L O G Y
Tsunami
Boulders
An island’s mysterious rocks were
likely deposited by giant waves
The dense vegetation of Isla de Mona,
an uninhabited speck in the Caribbean,
hides car-sized boulders studded with cor-
als. Scientists first reported spotting them
in the early 1990s, but the strange rocks
slipped back into obscurity before anyone
investigated their origin. Now researchers
have revisited these behemoths and con-
cluded that massive tsunami waves,
launched by an underwater landslide,
heaved them from the sea.
Many of the boulders are visible from
the air, but most are difficult to reach from
the ground, says Bruce Jaffe, an oceanog-
rapher at the Pacific Coastal and Marine
Science Center in Santa Cruz, Calif. He
recalls picking his way around the island’s
poisonous plants, including one that can
cause blistering and temporary blindness.
“We’ve talked about going back with
hazmat suits,” he says.
During two trips, the
researchers visited more
than 50 boulders. The larg-
est was more than eight
meters long, and the rocks
were strewn over a wide
area up to 800 meters
inland. A storm probably
would not have deposited
them so far from the shoreline, Jaffe says; a
powerful tsunami must have been involved.
Ricardo Ramalho, a geologist at the Univer-
sity of Lisbon, who was not involved in the
research, agrees: “I would find it surprising
if it was the work of a storm,” he says.
Team member Pedro Israel Matos Llavo-
na, a geoscientist at the University of Massa-chusetts Amherst, pored over maps of the
nearby seafloor and found evidence of
something that could have triggered such
a tsunami: a jagged depression nearly four
kilometers wide, the relic of a long-ago
underwater landslide. Matos
Llavona simulated the land-
slide and found it would
have sent 10-meter-high tsu-
nami waves crashing onto
Isla de Mona—with enough
force to heave many-ton
boulders far inland. The
researchers presented
these results at the 2020
American Geophysical Union Fall Meeting.
Next the team plans to pinpoint more
of these boulders by flying a drone to de -
tect the heat that they emit after a day
in the sun. The researchers suggest that
tracing how the rocks are scattered may
help scientists detect tsunami signatures
elsewhere. — Katherine Kornei